The present invention relates to a power supply and an arc processing power supply including an inverter circuit that converts DC power into predetermined high frequency AC power.
A power supply for an arc processing machine or the like generates DC power from input AC power and converts the DC power to high frequency AC power by performing a switching operation with an inverter circuit. The power supply uses a welding transformer to adjust the voltage of the high frequency AC power. Then, a downstream circuit converts the voltage-adjusted high frequency AC power to DC output power that is suitable for arc welding or the like. The output voltage is adjusted by controlling the switching operation of the inverter circuit.
Japanese Laid-Open Patent Publication No. 2006-280120 describes an example of a power supply including an inverter circuit configured by two pairs of switching elements. Two control pulse signals are provided to the two pair of switching elements so that the pairs of switching elements are alternately activated and deactivated. Phase shift modulation (PSM) control is performed so that the phase of the control pulse signal provided to the first pair of switching elements is shifted from the phase of the control pulse signal provided to the second pair of switching elements. More specifically, the phase difference between the control pulse signal provided to the first pair and the control pulse signal provided to the second pair is controlled (ON pulse width is fixed) to adjust the time during which corresponding switching elements of the first and second pairs are simultaneously activated. This adjusts the average voltage of the high frequency AC power output from the inverter circuit. In other words, the output voltage of the power supply is adjusted.
Further, the power supply includes a rectification-smoothing circuit that converts input AC power to DC power. An auxiliary switching element is arranged along a power line between the rectification-smoothing circuit and the inverter circuit. Activation and deactivation of the auxiliary switching element is performed in cooperation with the switching elements of the inverter circuits, and charging and discharging of the auxiliary capacitor is performed accordingly. This is referred to as soft switching control and reduces switching losses in the auxiliary switching element and the inverter circuit switching elements.
When a typical pulse width modulation (PWM) control is performed so that the output voltage of the power supply is adjusted to be extremely small, a control pulse signal having an extremely narrow ON pulse width is set. This may hinder activation of a switching element and result in an unstable output or biased magnetization.
In contrast, when PSM control is performed so that the output voltage of the power supply is adjusted to be extremely small, the two control pulse signals have a large phase difference and a sufficiently wide ON pulse width. In this case, the pulse width is fixed so that the maximum output can be generated. This ensures activation of the switching elements and avoids the problems of unstable output or biased magnetization.
However, since the ON pulse width is set to be sufficiently wide so that the maximum output can be generated, when the phase difference between two control pulse signals becomes large, after one of the two switching elements is deactivated, the activated time of the other one of the two switching elements becomes long. In particular, when extremely small output power is required, the activation time of the other switching element becomes long. This lengthens the time during which return current is produced in the inverter circuit. Conduction loss caused by the return current is increased. Thus, power is not efficiently used.
It is an object of the present invention to provide a power supply and an arc processing power supply that stabilizes output when the output is low and contributes to efficient power consumption.